Plastics are widely used in the manufacture of containers. These plastic containers generally have chemically inert nonporous surfaces with low surface energy. This makes them non-receptive to bonding with inks, adhesives, coatings, and other substrates.
The surface energy of a solid varies with its chemical make-up. Metals and glass have a high surface energy and are easier to bond; whereas plastics have a lower surface energy and are harder to bond. The following table shows the surface energies of a number of metals and polymers.
MaterialSurface free energy at 20° C. (in mJ/m2)Aluminium840Copper1100Polyethylene-linear35.7Polypropylene-isotactic30.1Polystyrene40.7Polyvinylchloride41.5Polymethylmethacrylate41.1Polyethyleneterephthalate44.6Polycarbonate34.2
It is known in the art to increase the surface energy of low surface energy materials by applying surface modification techniques which can change the chemical composition of the surface to increase the surface energy. These techniques include UV/ozone, flame, corona or plasma treatment and chemical oxidation.
Plastic containers are widely used in a variety of applications. Squeezable plastic containers are often used to package viscous slurry-like or paste-like fluids, such as ketchup, shampoo etc. Often, containers holding highly viscous fluid contents are stored in an inverted position (top-down) to quickly discharge the contents and to minimize the amount of product that remains in the container once it has been emptied. Nonetheless, it is generally difficult to quickly dispense viscous fluids from plastic containers and often a substantial amount of product remains stuck to the inside of the container once the container has been emptied.
It is known to improve the dispensing properties of plastic containers by applying a suitable internal coating.
US 2016/0039557 describes a blow-formed container having a liquid layer formed in an amount of not more than 10 g/m2 on the whole inner surface of the stretched portion thereof. An oleophilic liquid is used for aqueous content, and water or a hydrophilic liquid is used for the oily content. Silicone oil, fatty acid ester of glycerin, fluid paraffin, and edible fat and oil are mentioned as examples of oleophilic liquids that may be used.
It is also known to improve the dispensing properties of plastic containers by adding slip-improving components to the polymer resins from which containers are formed. US 2010/092621 describes a multi-layer plastic container for a non-oily content having at least a polyolefin resin layer on an inner surface thereof and for being filled with the non-oily content, wherein the polyolefin resin layer on the inner surface of the container contains an aliphatic amide in an amount of not less than 500 ppm but less than 4000 ppm.
US 2014/0332437 describes a manufacturing method of a food container having oxygen barrier properties, the method comprising the steps of:                preparing a container made of a plastic material;        performing an oxygen plasma treatment on a surface of the container;        depositing a buffer thin layer having a thickness of 5 to 30 nm on the surface of the container; and        depositing an oxygen barrier thin layer on the buffer thin layer.        
The buffer thin layer may be formed of hexamethyldisiloxane (HMDSO) or silicon (Si).
The oxygen barrier thin layer may be formed of silicon oxide.
US2010092621A (Toyo Seikan Kaisha Ltd) discloses a polyethylene container which enables a non-oily viscous content such as ketchup to favorably run down when the container is inverted. The solution lies in at least one polyethylene resin layer on the inner surface containing an aliphatic amide not less than 500 ppm but less than 4000 ppm.